Effective parasitic capacitance minimization for micro ablation electrode

ABSTRACT

A flexible catheter has an ablation electrode disposed in its distal segment. The ablation electrode a cavity formed in its external surface, a microelectrode configured to fit into the cavity, a conductive wire lead connecting the microelectrode to receiving circuitry, and an electrical shield surrounding the wire lead. A power generator is connected to the ablation electrode and the electrical shield in a generator circuit. A back patch electrode adapted to contact with the subject is connected in the generator circuit. The microelectrodes can be active while energizing the ablation electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/063,456, filed Oct. 14, 2014, which is herein incorporated byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to medical instruments for electrical applicationto the body. More particularly, this invention relates to improvementsin medical ablation catheters.

2. Description of the Related Art

Radiofrequency (RF) ablation of the heart is a procedure that is widelyused to correct problematic cardiac conditions, such as atrialfibrillation. The procedure typically involves insertion of a catheterhaving an electrode into the heart, and ablating selected regions withinthe heart with RF energy transmitted via the electrode. Capacitiveeffects can interfere with electrophysiologic signals when power istransmitted to an ablation electrode.

U.S. Patent Application Publication No. 2014/0155758, entitled LowCapacitance Endoscopic System proposes an endoscopic system havingdistal sensors, in which the capacitance of the sensor system relativeto earth ground maintains current leakage to a level that meets acardiac float rating. During sensing, power can be transmitted to thesensor via a power transmission line from a groundreferenced powersource, and data signals can be transmitted to the sensor via a datasignal transmission line from a processing circuit at a proximate end ofthe endoscopic shaft. In response to electromagnetic interferenceproximate the remote surgical site, induced voltages level changes inthe data signal transmission line and the power transmission aresubstantially equalized.

Arrangements wherein an ablation electrode in the catheter is inproximity to microelectrodes are known, for example, from U.S. PatentApplication Publication No. 2013/0190747, which discloses an ablationcatheter having a tissue ablation electrode and a plurality ofmicroelectrodes distributed about the circumference of the tissueablation electrode and electrically isolated therefrom. The plurality ofmicroelectrodes define a plurality of bipolar microelectrode pairs. Inthis arrangement mapping microelectrodes are disposed near the ablationtip electrode to allow the center of mapping or pacing to be insubstantially the same location as the center of ablation. It isasserted that the microelectrodes can advantageously provide feedback onelectrode contact and tip electrode orientation within the heart.

SUMMARY OF THE INVENTION

There is provided according to embodiments of the invention an apparatusincluding a flexible catheter adapted for insertion into a heart of aliving subject, an ablation electrode disposed at the distal segment ofthe catheter to be brought into contact with a target tissue in theheart. The ablation electrode has a cavity formed in its externalsurface, a microelectrode configured to fit into the cavity, aconductive wire lead connecting the microelectrode to receivingcircuitry, and an electrical shield surrounding the wire lead.

A further aspect of the apparatus includes a generator circuitconnecting a power generator to the ablation electrode and theelectrical shield, and a back patch electrode adapted to contact withthe subject and connected in the generator circuit.

According to another aspect of the apparatus, the electrical shieldincludes a coaxial layer, and a dielectric layer disposed between thecoaxial layer and the wire lead.

According to one aspect of the apparatus, the electrical shield alsoincludes an insulating jacket that overlies the coaxial layer.

According to a further aspect of the apparatus the microelectrode iscontoured, located and oriented to conform to a curvature of theablation electrode.

According to yet another aspect of the apparatus, the ablation electrodehas a cylindrical portion, wherein the cavity includes a plurality ofcavities formed in the cylindrical portion.

According to still another aspect of the apparatus, the ablationelectrode has a distal annular portion, wherein the cavity includes aplurality of cavities formed in the annular portion and a plurality ofmicroelectrodes disposed therein.

According to an additional aspect of the apparatus, the microelectrodeis linked to a thermocouple that provides a signal representative of atemperature of the microelectrode.

There is further provided according to embodiments of the invention amethod, which is carried out by inserting a flexible catheter into aheart of a living subject. The catheter has an ablation electrodedisposed at the distal segment of the catheter. A cavity is formed inthe external surface ablation electrode and a microelectrode fitted intothe cavity. A conductive wire lead connects the microelectrode andreceiving circuitry. An electrical shield surrounds the wire lead. Apower generator connects the ablation electrode and the electricalshield in a generator circuit. The method is further carried out byconnecting a back patch electrode to the subject and to the generatorcircuit. The method is further carried out by contacting the ablationelectrode with a target tissue in the heart, and while receiving signalsfrom the microelectrode in the receiving circuitry energizing theablation electrode to ablate the target tissue.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the detailed description of the invention, by way of example, whichis to be read in conjunction with the following drawings, wherein likeelements are given like reference numerals, and wherein:

FIG. 1 is a pictorial illustration of a system, which is constructed andoperative in accordance with a disclosed embodiment of the invention;

FIG. 2 is a schematic diagram illustrating assembly of distal end of acatheter in accordance with an embodiment of the invention;

FIG. 3, which is a schematic sectional view of the catheter shown inFIG. 2 taken through its axis of symmetry in accordance with anembodiment of the invention;

FIG. 4 is a schematic diagram of an embodiment of the distal segment ofan ablation catheter, in accordance with an embodiment of the invention;

FIG. 5 is an electrical schematic of the arrangement shown in FIG. 4 inaccordance with an embodiment of the invention showing respectiveparasitic current distribution because of the ablation signal; and

FIG. 6 is an electrical schematic showing a version of the arrangementwithout the guard protection in accordance with the prior art.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the various principles ofthe present invention. It will be apparent to one skilled in the art,however, that not all these details are necessarily needed forpracticing the present invention. In this instance, well-known circuits,control logic, and the details of computer program instructions forconventional algorithms and processes have not been shown in detail inorder not to obscure the general concepts unnecessarily.

Documents incorporated by reference herein are to be considered anintegral part of the application except that, to the extent that anyterms are defined in these incorporated documents in a manner thatconflicts with definitions made explicitly or implicitly in the presentspecification, only the definitions in the present specification shouldbe considered.

The terms “link”, “links”, “couple” and “couples” are intended to meaneither an indirect or direct connection. Thus, if a first device islinked to a second device, that connection may be through a directconnection, or through an indirect connection via other devices andconnections.

Overview.

Turning now to the drawings, reference is initially made to FIG. 1,which is a pictorial illustration of a system 10 for evaluatingelectrical activity and performing ablative procedures on a heart 12 ofa living subject, which is constructed and operative in accordance witha disclosed embodiment of the invention. The system comprises a catheter14, which is percutaneously inserted by an operator 16 through thepatient's vascular system into a chamber or vascular structure of theheart 12. The operator 16, who is typically a physician, brings thecatheter's distal tip 18 into contact with the heart wall, for example,at an ablation target site. Electrical activation maps may be prepared,according to the methods disclosed in U.S. Pat. Nos. 6,226,542, and6,301,496, and in commonly assigned U.S. Pat. No. 6,892,091, whosedisclosures are herein incorporated by reference. One commercial productembodying elements of the system 10 is available as the CARTO® 3 System,available from Biosense Webster, Inc., 3333 Diamond Canyon Road, DiamondBar, Calif. 91765. This system may be modified by those skilled in theart to embody the principles of the invention described herein.

Areas determined to be abnormal, for example by evaluation of theelectrical activation maps, can be ablated by application of thermalenergy, e.g., by passage of radiofrequency electrical current throughwires in the catheter to one or more electrodes at the distal tip 18,which apply the radiofrequency energy to the myocardium. The energy isabsorbed in the tissue, heating it to a point (typically about 50° C.)at which it permanently loses its electrical excitability. Whensuccessful, this procedure creates non-conducting lesions in the cardiactissue, which disrupt the abnormal electrical pathway causing thearrhythmia. The principles of the invention can be applied to differentheart chambers to diagnose and treat many different cardiac arrhythmias.

The catheter 14 typically comprises a handle 20, having suitablecontrols on the handle to enable the operator 16 to steer, position andorient the distal end of the catheter as desired for the ablation. Toaid the operator 16, the distal portion of the catheter 14 containsposition sensors (not shown) that provide signals to a processor 22,located in a console 24. The processor 22 may fulfill several processingfunctions as described below.

Ablation energy and electrical signals can be conveyed to and from theheart 12 through one or more ablation electrodes 32 located at or nearthe distal tip 18 via cable 34 to the console 24. Pacing signals andother control signals may be conveyed from the console 24 through thecable 34 and the electrodes 32 to the heart 12. Sensing electrodes 33,also connected to the console 24 are disposed between the ablationelectrodes 32 and have connections to the cable 34.

Wire connections 35 link the console 24 with body surface electrodes 30and other components of a positioning sub-system for measuring locationand orientation coordinates of the catheter 14. The processor 22 oranother processor (not shown) may be an element of the positioningsubsystem. The electrodes 32 and the body surface electrodes 30 may beused to measure tissue impedance at the ablation site as taught in U.S.Pat. No. 7,536,218, issued to Govari et al., which is hereinincorporated by reference. A temperature sensor (not shown), typically athermocouple or thermistor, may be mounted on or near each of theelectrodes 32.

The console 24 typically contains one or more ablation power generators25. The catheter 14 may be adapted to conduct ablative energy to theheart using any known ablation technique, e.g., radiofrequency energy,ultrasound energy, and laser-produced light energy. Such methods aredisclosed in commonly assigned U.S. Pat. Nos. 6,814,733, 6,997,924, and7,156,816, which are herein incorporated by reference.

In one embodiment, the positioning subsystem comprises a magneticposition tracking arrangement that determines the position andorientation of the catheter 14 by generating magnetic fields in apredefined working volume and sensing these fields at the catheter,using field generating coils 28. The positioning subsystem U.S. Pat. No.7,756,576, which is hereby incorporated by reference, and in theabove-noted U.S. Pat. No. 7,536,218.

As noted above, the catheter 14 is coupled to the console 24, whichenables the operator 16 to observe and regulate the functions of thecatheter 14. Console 24 includes a processor, preferably a computer withappropriate signal processing circuits. The processor is coupled todrive a monitor 29. The signal processing circuits typically receive,amplify, filter and digitize signals from the catheter 14, includingsignals generated by sensors such as electrical, temperature and contactforce sensors, and a plurality of location sensing electrodes (notshown) located distally in the catheter 14. The digitized signals arereceived and used by the console 24 and the positioning system tocompute the position and orientation of the catheter 14, and to analyzethe electrical signals from the electrodes.

Typically, the system 10 includes other elements, which are not shown inthe figures for the sake of simplicity. For example, the system 10 mayinclude an electrocardiogram (ECG) monitor, coupled to receive signalsfrom one or more body surface electrodes, in order to provide an ECGsynchronization signal to the console 24. As mentioned above, the system10 typically also includes a reference position sensor, either on anexternally-applied reference patch attached to the exterior of thesubject's body, or on an internally-placed catheter, which is insertedinto the heart 12 maintained in a fixed position relative to the heart12. Conventional pumps and lines for circulating liquids through thecatheter 14 for cooling the ablation site are provided. The system 10may receive image data from an external imaging modality, such as an MRIunit or the like and includes image processors that can be incorporatedin or invoked by the processor 22 for generating and displaying images.

Microelectrode Catheter Tip.

Reference is now made to FIG. 2, which is a schematic diagramillustrating an assembly of distal end 37 of a catheter in accordancewith an embodiment of the invention. An isolated guard shield is placedover the microelectrode wires and connected to an ablation electrode toequalize potentials. This arrangement minimizes stray capacitance thatmay conduct current to other shaft electrodes or to system ground. Aninsertion tube terminates in the distal end 37, which is formed from abiocompatible conductor, such as platinum, palladium, gold, iridium, oran alloy of the aforementioned, and which has an axis of symmetry 39. Atleast one cavity 41 is formed in the cylindrical region of externalsurface 74, and at least one cavity 43 is formed in the curved annularregion of the external surface. The embodiment described hereincomprises three cavities 41, which are distributed symmetrically withrespect to axis of symmetry 39, and three cavities 43 are alsodistributed symmetrically with respect to the axis of symmetry 39.However, these numbers and distributions are purely by way of example.Embodiments of the present invention may have different numbers ofcavities, and different distributions of the cavities, from thosedescribed herein. As described below, each cavity 41 is configured toaccept and mate with a respective microelectrode 45 and each cavity 43is configured to accept and mate with a respective microelectrode 47.

Reference is now made to FIG. 3, which is a schematic sectional view ofthe distal end 37 (FIG. 2) taken through the axis of symmetry 39 inaccordance with an embodiment of the invention. Each microelectrode 45receives at least one conductive wire 49. Similarly, each microelectrode47 receives at least one conductive wire 49. The conductive wires 49 aretypically insulated so that they are electrically isolated from the wallof the distal end 37. A shield 51, described in further detail below,surrounds the conductive wire 49 leading from the microelectrode 47. Ineach case, the conductive wires 49 are connected to the microelectrodes45, 47, typically by soldering and/or welding. Each conductive wire 49is conveyed circuitry in the console 24 (FIG. 1), enabling potentialsgenerated at the different microelectrodes to be measured.

Further details of the distal end 37 are found in commonly assignedcopending application Ser. No. 14/279,682, which is herein incorporatedby reference.

Reference is now made to FIG. 4, which is a schematic diagram of anembodiment of the distal segment of an ablation catheter 53 inaccordance with an embodiment of the invention. The catheter 53 has anablation electrode 55 (M1) at tip 57 connected to a wire 59 thatsupplies power from a system console (not shown). A microelectrode 61 onthe ablation electrode 55 can be a sensing electrode, e.g., by linkageto a thermocouple 63. The microelectrode 61 is connected to the systemconsole by a wire lead 65. A ring electrode 67 (M2) may provide othersensor information, e.g., an electrogram, and is connected to the systemconsole by a wire 69. An electrical shield 71, e.g., a coaxial layer,surrounds the wire lead 65, and may be separated from the wire lead 65by a dielectric layer 73. An optional external insulating jacket (notshown) may overly the electrical shield 71.

Reference is now made to FIG. 5, which is an electrical schematic of thearrangement shown in FIG. 4 in accordance with an embodiment of theinvention. ECG electrodes 67, 75 and ablation electrode 55 mounted onthe ablation catheter shaft are connected to a power generator 77 viagenerator circuit 79 via a back patch 81 through the patient body.Resistances of the patient's body are represented by resistors 83between the ECG electrodes 67, 75 and the back patch 81. Resistance ofthe patient's body between the ablation electrode 55 and the back patch81 is represented by resistor 85. The back patch 81 may be implementedby a conventional skin patch that is connectable to the patient's body.Parasitic capacitances 87, 89 of ECG electrodes 67, 75 and parasiticcapacitance 91 of the microelectrode 61 are separated by the electricalshield 71 connected to the generator circuit 79 via the electricaljunction 93. The electrical shield 71 provides an envelope withpotential equal to the potential of ablation electrode 55 (M1) so thatthe differential potential between ablation electrode 55 (M1) andmicroelectrode 61 and hence the current flowing therebetween isminimized. Interference with the signal produced by microelectrode 61 isconsequently minimized. Leakage current from ECG electrodes 67, 75 dueto parasitic capacitances 87, 89 is supplied by the ablation electrode55 (ml) through junction 93 and the electrical shield 71 (and not bymicroelectrode 61).

Reference is now made to FIG. 6, which is an electrical schematicshowing a version of the elements of the circuit shown in FIG. 5, inaccordance with the prior art. In the absence of the electrical shieldshown in FIG. 5 capacitive leakage current can flow between themicroelectrode and the afferent limb of the generator circuit via theresistors 83 (representing the patient's body) and the back patch 81.This can accordingly cause microablation on microelectrode 61 anddistort readings taken from the microelectrode 61, e.g., temperaturemeasurements via the linked thermocouple 63 (FIG. 4).

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of the present inventionincludes both combinations and sub-combinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofthat are not in the prior art, which would occur to persons skilled inthe art upon reading the foregoing description.

1.-8. (canceled)
 1. A method, comprising the steps of: inserting aflexible catheter into a heart of a living subject, the catheter havinga distal segment, an ablation electrode disposed at the distal segment;the ablation electrode having an external surface; forming a cavity inthe external surface; fitting a microelectrode into the cavity;connecting a conductive wire lead to the microelectrode and to receivingcircuitry; placing an electrical shield surrounding the wire lead;contacting the ablation electrode with a target tissue in the heart; andwhile receiving signals from the microelectrode in the receivingcircuitry energizing the ablation electrode to ablate the target tissue.2. The method according to claim 1, further comprising the steps of:connecting a power generator to the ablation electrode and theelectrical shield in a generator circuit; and connecting a back patchelectrode to the subject and to the generator circuit.
 3. The methodaccording to claim 1, wherein the electrical shield comprises: a coaxiallayer; and a dielectric layer disposed between the coaxial layer and thewire lead.
 4. The method according to claim 3, wherein the electricalshield further comprises an insulating jacket that overlies the coaxiallayer.
 5. The method according to claim 1, wherein the external surfaceof the ablation electrode has a curvature, and the microelectrode iscontoured, located and oriented to conform to the curvature.
 6. Themethod according to claim 1, wherein the ablation electrode has acylindrical portion, wherein the cavity comprises a plurality ofcavities formed in the cylindrical portion.
 7. The method according toclaim 1, wherein the ablation electrode has a distal annular portion,wherein the cavity comprises a plurality of cavities formed in theannular portion and a plurality of microelectrodes disposed therein. 8.The method according to claim 1, wherein the microelectrode is linked toa thermocouple, wherein the thermocouple provides a signalrepresentative of a temperature of the microelectrode.